1. Field of the Invention
The present invention relates to a cathode-ray tube having a face plate including an optical interference filter provided between the inner surface of a face plate for cathode-ray tube of a television set and a phosphor layer coated on the inner surface of the face plate for cathode-ray tube and consisting of a plurality of alternately stacked high- and low-refractive index material layers and, more particularly, to a projecting cathode-ray tube for enlarging and projecting an image in front of the cathode-ray tube on a screen via a projection lens arranged opposite to a phosphor screen and a process for producing the same.
2. Description of the Prior Art
A conventional projecting television set and a cathode-ray tube for use in the television set will be described below with reference to FIGS. 10A and 10B.
As shown in FIG. 10A, a conventional projecting television set for projecting an image on a screen in an enlarged scale includes three projecting cathode-ray tubes 71a, 71b and 71c for respectively emitting blue, green and red light, an optical system for focusing the light from the cathode-ray tubes 71a, 71b and 71c by using lenses 72a, 72b and 72c, respectively, and a screen 73 on which the respective images are projected. As shown in FIG. 10B, in a certain type of projecting cathode-ray tube 71 used in such a television set, a phosphor layer 8 is coated on the inner surface of a face plate 1 for cathode-ray tube, and an optical interference filter 7 consisting of alternate layers such as a short path edge filter is provided between the phosphor layer 8 and the inner surface of the face plate 1 for cathode-ray tube. An optical interference filter is used which has, as its cutoff wavelength, a wavelength .lambda..sub.O slightly longer than a center wavelength .lambda. of an emission spectrum of a phosphor as shown in FIG. 11. JP-A 273837/1986 discloses a face plate for cathode-ray tube having an optical interference film of this type.
In addition to specific characteristics as an optical interference filter, an optical interference filter to be formed on the inner surface of a face plate for cathode-ray tube must have a function of efficiently, externally transmitting light emitted from a phosphor upon radiation of an electron beam. In addition, a phosphor layer must have a strong adhesive force with respect to the optical interference filter and must be coated without forming pinholes.
In order to efficiently, externally transmit light emitted from a phosphor, an optical interference filter must be prevented from being colored to be blackened by a browning phenomenon caused by a radiated electron beam. In the optical interference filter according to the above prior art, however, if a layer in contact with the phosphor layer consists of SiO.sub.2, the optical interference filter is colored upon continuous radiation of an electron beam.
In order to stably obtain an adhesive force of a phosphor layer to be coated on an optical interference filter and to obtain a phosphor coat surface without pinholes, the surface of the optical interference filter must be cleaned before coating In order to coat the interference filter on a face plate for cathode-ray tube with a strong adhesive force, the surface of the face plate for cathode-ray tube must be cleaned before coating. However, when an aqueous solution containing hydrofluoric acid used as a strong cleaning solution for a face plate for cathode-ray tube before formation of an optical interference filter is used to clean the surface of an optical interference filter before coating of a phosphor, if a layer in contact with the phosphor layer consists of SiO.sub.2 as in the prior art, the optical interference filter may be partially dissolved or peeled. If, however, a neutral detergent or an aqueous solution containing a mineral acid such as hydrochloric acid is used to clean the surface of an optical interference filter, the surface cannot be sufficiently cleaned to make it difficult to avoid formation of small pinholes in a coated phosphor layer.